Plant Speciation

1
Plant Speciation Evolution (PBIO 475/575)

• Selection and Adaptation

2
Selection

• "Selection is the statistically consistent
  differential survival and/or reproduction of two
  or more classes of entities" (Futuyma 1979)
• selection occurs when some individuals express
  better survival and reproduction ( more
  fitness), on average, than other individuals
• only requires one more progeny, on average, over
  lifespan of a genotype!

3
Levels of Selection
Futuyma (1979)
4
Group Selection

• Forces that benefit an entire population at the
  potential expense of individuals--counter to
  individual selection
• Investigated in animals, maybe no plant analog
• Some evolutionists argue against the model and
  its importance, suggesting "altruistic" alleles
  would increase in populations by random drift and
  against selective forces
• classic example warning call of a bird that
  "saves" the flock from predation

5
Kin Selection

• Another model of selection with probably no
  analog in plants
• Higher chance of survival of an allele, but not
  directly by increasing survival of individual
  inheriting it rather, progeny ("kin") that also
  carry the new allele survive

6
Kin Selection

• e.g., parental care in animals--doesn't benefit
  mother directly but increases chances of survival
  of offspring
• may represent a case of "inclusive
  fitness"--reproductive success of an individual
  plus all the allele copies carried by its progeny
  to whom the individual extends "care"

7
Frequency-dependent Selection

• In addition to heterozygote superiority, a model
  of selection that can maintain a polymorphism
• If genotype is most "advantageous" when rare(r),
  it will be maintained at low levels
• e.g., self-incompatibility in plantsalleles with
  lower frequency will increase chances for
  successful cross-pollination and seed set

8
Frequency-dependent Selection

• e.g., fish predation on color morphs of a true bug

Futuyma (1979)
9
Sexual Selection

• Competition among gametes for fusion
• Is not equivalent to natural selection or really
  a component of it, and is antagonistic to natural
  selection but worth mentioning (you'll see it in
  the literature)
• Overall fitness of organisms with a complete life
  cycle involving 2 components or life
  stage--vegetative success through growth and
  sexual success through fertilization

10
Sexual Selection

• Sexual selection needed as an explanation for
  cases where vegetative and sexual success are
  antagonistic
• Life cycle is therefore balanced between sexual
  selection and natural selection
• e.g., the relative cost/benefit vegetative vs.
  reproductive structures in plants with
  clonal-sexual mixed breeding systems

11
Selectional Mosaics

• Useful to view plants and other organisms as
  "modularized selectional mosaics"
• Different phenotypic traits at different
  selection levels may respond to selective forces
  in very different ways (or experience NO
  selection at all!)
• Not all traits or suites of traits will be
  capable of responding to changing conditions over
  life span of the organism

12
Selectional Mosaics

• Highlights the (perhaps more realistic) view that
  fittest genotypes are those with collective
  phenotype that is least compromised by conditions
  ranging over the entire life span of the
  organism, and still capable of producing at least
  one more progeny on average than any other
• Critical to evaluate cumulative reproductive
  output over lifespan of genotype (virtually never
  done)

13
Selectional Mosaics

• "Doing the best with what you've got
• Are we entirely missing the point by pursuing
  investigations of maximal performance? Do we
  really understand much about selection at all
  then?

14
Fitness Landscapes

• Wright's concept of a "fitness landscape"
• Landscape with adaptive peaks harboring most fit
  genotypes/phenotypes
• Valleys harbor less fit genotypes/phenotypes
• Selection eliminates the less fit individuals in
  the valley over time

Niklaus (1997)
15
Adaptive Walk

• Movement of a population of individuals on the
  fitness landscape over time
• Both random forces (e.g., drift) and non-random
  ones (natural selection) act directly on
  population, specifically on the
  phenotype--gtlandscape can be represented as all
  possible phenotypes

16
Adaptive Walk

• Genotypic/phenotypic composition of population
  gradually moves toward and up a peak over time
• Some peaks are locally adaptive in terms of
  fitness but are less adaptive than other
  peaks--can population move back through
  maladaptive valley to get to another "better"
  peak?

17
Fitness Landscape Example

• Graph of 2 linked chromosomal inversions in the
  Australian grasshopper, Moraba scurra

Futuyma (1979)
18
Responses to Changing Environments

• Alternative phenotypic shifts to changing
  conditions

Niklaus (1997)
19
Responses to Changing Environments

• Sexual shifts in protists in response to change

Niklaus (1997)
20
Homeostasis

• Genetic homeostasis--equilibration of the genetic
  composition of a population, resistant to sudden
  changes
• Developmental homeostasis--genotype of a
  population produces the proper adaptive phenotype
  regardless of developmental perturbations
• e.g., symmetry in structures and organisms
• e.g., suggested by phenotypic uniformity of
  certain fern species over 30-100 million years!

21
Genetic Assimilation

• Trait is initially only environmentally induced
• Over time, trait becomes genetically fixed
• e.g., locally adaptive traits in ecological
  races--remember Clausen et al.'s transplant
  experiments with Potentilla and Achillea?
• Not many studies on this--not clear how important
  this is, but it may be

22
Canalization

• Channeling of a potentially broad phenotypic
  response into a narrow genetic/developmental
  pathway
• Suggested to have evolved not because a trait is
  most adaptive but to achieve a developmental
  balance
• e.g., bristle number on flies
• e.g., petal number in many plant genera, evening
  primrose (Onagraceae) and mustard (Brassicaceae)
  families, most other more advanced angiosperm
  lineages

23
Adaptation in Theory

• Broad definition of adaptation--process by
  which modifications to organisms make them
  function more successfully (more fit) in a given
  environment
• Specific categories of adaptationnot always
  sharply distinct
• Aptation--any structure that increases fitness
  (generic, underlying term)
• Adaptation in the narrow sense--aptation
  developed through natural selection for its
  current use

24
Adaptation in Theory

• Specific categories of adaptation (cont.)
• Exaptation--developed characteristic that is
  later co-opted for a new use
• e.g., feathers in birds may have originally been
  essential for thermoregulation, later became used
  for flight
• Overlaps somewhat with preadaptation
• Preadaptation--structures or traits involved in
  one function may be utilized for an unrelated
  one, and later modified by natural selection to
  better profit the second function
• e.g., climbing, parachuting and gliding in
  ancestors of birds were each precursor steps in
  the evolution of flight in birds

25
Adaptation in Theory

• Specific categories of adaptation (cont.)
• Exaptation (cont.)
• Gould and others stress distinguishing the
  present utility of a structure modified by
  natural selection from its original role
• May be morphological, physiological or behavioral
• e.g., in plants, different steps in evolution of
  wind pollination
• tall plants to catch wind
• fewer flower parts
• reduced parts
• drooping flowers
• extruded dangling stamens
• more pollen

26
Adaptation in Theory

• Single traits vs. suites of traits
• Traits in different structures/organs may
  ultimately become a tightly linked suite
  (adaptive syndrome)
• Establishment of one successful trait may foster
  a cascade of others
• The same trait or syndrome may arise more than
  once within a lineage (parallelism) or in very
  distantly related lineages (convergence)

27
Controversies Over Adaptation

• "Adaptationist Program--all features are the
  direct or indirect result of natural selection
• Does not accommodate alternatives, e.g., drift,
  catastrophic events
• Believed by others to be seriously flawed
• Not supported by evidence available in all
  cases--recall that many traits are "selectively
  neutral" or even disadvantageous for bearer
  temporarily, but may become advantageous later
  with changing conditions

28
Controversies Over Adaptation

• Theory of neutral evolution
• Antithesis of Adaptationist Program
• Phenotypic change based purely on mutation,
  random fixation or loss of alleles at molecular
  level
• Proponents argue that phenotypic change arises
  and accumulates primarily from microevolutionary
  processes alone
• Sometimes admit that speciation and
  macroevolutionary processes may also require
  natural selection

29
Controversies Over Adaptation

• In most groups, probably both processes occur, as
  well as wholly external processes such as
  wholesale climatic or extraterrestrial impacts
• Debates from extremes have mostly died down, with
  biologists embracing both extremes (and points in
  between) as evidence demands

30
Analogy vs. Homology
Galium aparine

• Analogy--structure, organ or trait in two taxa
  with equivalent phenotype or function that arose
  from different body parts
• e.g., spines may arise from leaves, stipules,
  axillary branches
• e.g., apparent whorl of leaves in bedstraw
  (Galium) opposite leaves 2 modified stipules

31
Analogy vs. Homology
Cypripedium acaule

• Homology--structure, organ or trait in two
  different taxa that is phenotypically or
  functionally different but arose from the same
  body part
• e.g., pouch in lady slipper orchids, lemma in
  grasses, ray floret in sunflowers

Helianthus maximiliani
Bromus catharticus
32
Analogy vs. Homology

• e.g., different parts selected by humans from
  kale ancestor to produce different vegetables
  (Brassica oleracea)
• cabbage
• broccoli
• cauliflower
• brussel sprouts
• kohlrabi

Niklaus (1997)
33
Parallelism vs. Convergence

• Parallelism--The occurrence of independent
  evolutionary modifications of the same or a
  similar kind in closely related groups that have
  similar developmental programs inherited from a
  relatively recent common ancestor, and often
  confused with convergent evolution (Niklaus,
  1997)
• Convergence--The evolution by means of
  genetically different programs of superficially
  similar adaptive structures and habits in
  unrelated species" (Avers 1989)

34
Parallelism vs. Convergence

• No sharp boundary between these alternatives, or
  endpoints along the evolutionary continuum
• Parallelism generally refers to same trait
  among somewhat closely related (but not sister)
  species for instance, in same genus or family
  convergence represented as equivalent trait among
  very distantly related organisms in different
  lineages for instance, different orders
• Homoplasy--cladistic term referring to multiple
  independent origins of a trait in a phylogenetic
  tree encompasses both of the above

35
Parallelism vs. Convergence

• e.g., convergence of growth form and/or spiny
  leaves in desert plants in very distantly related
  plant groups
• cacti (Cactaceae)
• spurges (Euphorbiaceae)
• composites (Asteraceae)
• grape relatives (Vitaceae)
• milkweed relatives (Asclepiadaceae)

Niklaus (1997)
36
Bibliography

• Bell, G. 1997. Selection The mechanism of
  evolution. Chapman and Hall, New York. 699 pp.
• Futuyma, D. J. 1979. Evolutionary biology.
  Sinauer Associates, Inc., Sunderland,
  Massachusetts. 565 pp.
• Niklaus, K. J. 1997. The evolutionary biology of
  plants. University of Chicago Press, Chicago,
  Illinois. 449 pp.